The present invention relates, in general, to the field of pulse width modulation (PWM) controllers for an electrical load. More particularly, the present invention relates to a PWM controller for a DC motor in which motor speed and PWM frequency are each respectively controlled with a single ground referenced input utilizing a dual differential input topology.
Conventional PWM controller integrated circuits (IC's) generally employ a reference voltage and a fixed ramp to control power by pulse width modulation. This technique has proven satisfactory for closed loop systems such as those encountered in switching power supplies. However, in applications such as motor drives, this conventional approach has proved overly cumbersome and existing IC's necessitate complex and excessive arrays of external devices.
In most applications, in particular those involving electromechanical electrical loads such as DC motors, it would be highly desirable to merely connect a potentiometer to the input of a PWM IC and the motor to the output without the addition of numerous and complex external components. Still further, the output power of the device should be proportional to a single ground referenced input without the need for scaling found in conventional controls which have heretofore consisted primarily of a combination of signal IC's, discrete resistors and capacitors, discrete power transistors or discrete power diodes. By utilizing a duty cycle proportional to a ground referenced input, the control function can then be accomplished by merely adding a potentiometer or a digital-to-analog (D/A) converter to the IC front end.
Other desirable features of a PWM controller would include having a duty cycle fully variable from 0% to 100%. Heretofore, the duty cycle range in most conventional PWM IC's is deliberately restricted. However, and as is particularly the case with motor drives, it is desirable to fully stop the motor in addition to running it at full speed. It is further highly desirable to control the frequency with a single ground referenced input voltage. Frequencies on the order of 200 hertz to 1 kilohertz are a typical range. This feature is especially important in motor drives since motor torque and chatter are controlled by varying frequency. Additionally, current limiting and over-temperature protection would also be highly desirable in a PWM controller. In the former instance, stall current up to ten times the rated operating current or shorts across the motor terminals may be encountered. Over-temperature shutdown can also serve to protect both the drive and the motor when such conditions are present.
Still further, a PWM controller should have a standby current drain less than one milliamp or, more preferably, 100 microamps. A negligibly low current drain allows the motor to be turned off via the control function of a potentiometer, D/A converter etc., without the necessity of utilizing an on/off switch and without significantly draining the power source, which may in some instances be a battery. Additionally, a pulse width modulation controller should be readily integrated onto a semiconductor substrate such that it would include a minimum number of I/O pins while simultaneously requiring a minimum number of external components. In many applications, and in particular portable battery powered motor applications, there may not be a convenient printed circuit (PC) board to absorb additional external components or a complex integrated circuit package.